a) Field of the Invention
The invention is directed to a resonator mirror with a saturable absorber which is formed of a plurality of semiconductor layers on a substrate for use in a solid-state laser resonator.
b) Description of the Related Art
WO 96/36906 A1 describes optical components for generating pulsed laser radiation which can be used as a resonator mirror. These resonator mirrors contain a stack or layer construction having a reflector and a saturable absorber. The position of the absorber layer in an ensemble of layers is utilized to compensate for the wavelength dependence based on the structure of the ensemble of coatings with the absorption given by the absorbing material for a given wavelength range (page 8, lines 29-35). It can be gathered from FIG. 3 and the accompanying description that this step should make it possible to maintain a reflectivity of almost 100% over a wavelength range of approximately 50 nm.
Further, a negative dispersion of the group velocity of the radiation waves in the laser resonator is to be achieved with the layer construction (page 11, lines 1-3). The aim of the arrangement of the saturable absorber within the layer construction is to integrate the characteristic of saturable absorption in the layer construction in an optimal manner in addition to the characteristic of a negative dispersion (page 11, lines 19-27). The curve of the intensity inside the ensemble of coatings is shown in FIG. 4 for four wavelengths within a wavelength range of 40 nm. It can be gathered from FIG. 8 and the accompanying description that the reflectivity of the optical components can be adjusted by means of the position of the saturable absorber in the ensemble of coatings.
Further, claim 4 states that the saturable absorber is arranged at a location where there is a high radiation intensity change and a high absorption change in a wavelength range. Therefore, a predetermined reflection curve is to be provided within a wavelength range in cooperation with the other layers (see claim 5). A uniformly high reflection factor is to be achieved over a wavelength range of 50 nm (FIGS. 3, 8a-e). An optimization criterion is to maintain an optimal saturably absorbing effect of these optical components. It can be gathered from the description that the optimum (page 11, lines 33 to 37) consists in maintaining laser pulses with extremely short pulse widths (less than 10 fs) and/or (page 15, lines 10 to 16) that the saturable absorber is placed at a location where the desired effect occurs: to maintain the desired broad band with respect to a desired wavelength range.
It is the primary object of the present invention to provide a resonator mirror of comparatively simple construction with a saturable absorber for use in a solid-state laser resonator that can be highly loaded with respect to power. The saturable absorber should generate laser pulses with a width in the range of 0.1 to 100 ps, wherein a predetermined peak output should be maintained as constant as possible.
The invention relates to a resonator mirror with a saturable absorber for a laser wavelength xcexL which is formed of a layer sequence of a plurality of semiconductor layers on a substrate, wherein a Bragg reflector formed of a plurality of alternately arranged layers comprising a first material with an index of refraction nH and a second material with a lower index of refraction NL is grown on a surface of the substrate. The resonator mirror is provided for use in a mode-synchronized solid-state laser resonator with an output power greater than 1 W, especially greater than 7 W.
According to the invention, the resonator mirror with a saturable absorber comprises the Bragg reflector grown on a substrate and a threefold layer which is grown on the latter and which acts as a saturable absorber for the laser wavelength xcexL, wherein a single quantum layer is embedded within two layers outside an intensity minimum for the laser radiation xcexL and the threefold layer has a combined optical thickness of             λ      L        2    .
The refractive indices of the respective materials for the laser wavelength xcexL are taken into account for determining the layer thickness of the single quantum layer, the two layers between which the latter is embedded, and the first and second layers for the Bragg reflector. The optical thickness is given by the air wavelength divided by the index of refraction n of the corresponding layer for the laser wavelength xcexL. In this connection, the refractive indices of the two layers enclosing the single quantum layer are not critical and can also be different for each of the two layers. It is important to maintain the total optical thickness       λ    L    2
of the threefold layer. The selection of materials for the threefold layer is therefore governed in particular by the material properties of the Bragg reflector, especially by the grating constants of the utilized materials which should be identical as far as possible. Identical grating constants allow a monocrystalline growth of the layers on a monocrystalline substrate with as few defects as possible. Monocrystalline layer systems are particularly advantageous because they have an especially small absorption. The selection of material for the single quantum layer and its thickness depends, in turn, on its saturably absorbing properties (band gap) for the laser wavelength and is not limited to the materials mentioned herein. Above all, the two layers must have a low absorption for laser wavelength xcexL and the property that they produce a permanent fixed connection with the single quantum layer and the layer system of the Bragg reflector.
The threefold layer will be referred to hereinafter as the layer having the saturably absorbing effect; only the single quantum layer with its band gap is the actual absorbing layer, but it is only capable of functioning due to the fact that it is embedded within the threefold layer in the manner described in the following.
In the present case, the single quantum layer is not subject to any resonance condition within a laser resonator. Its function is comparable with that of a dye absorber in a dye laser or solid-state laser. It must be stressed that ultra-short pulses in the femtosecond and milliwatt range which are desirable in communications technology should not be generated in this case. In practice, the resonator mirror is designed in such a way that a given high reflection factor for the laser wavelength xcexL is achieved with the smallest possible number of alternating individual layers, wherein a reflection factor of 98% is generally sufficient for laser operation. Including the threefold layer as saturable absorber, for example, only about 30 individual layers are required for a reflector with a saturably absorbing effect. This comparatively small number of individual layers requires a correspondingly low expenditure on manufacturing. What is more important, however, is that the comparatively small number of individual layers in combination with a corresponding control and management of the coating process leads to a very homogeneous layer construction vertical to the radiation direction of the laser light. This, in turn, enables the use of a comparatively slight focusing of the laser beam on the resonator mirror. The spot diameter on the resonator mirror can be more than 200 xcexcm in this case and can be expanded to approximately 5 mm, wherein a neat, constant mode synchronization of the laser is effected. These comparatively large spot diameters substantially reduce the power density at the resonator mirror. Typical values range from less than 100 kW/cm2 to about 2 kW/cm2 with respect to CW operation of the laser. However, in practice, operation is performed as close as possible to the load limit of the resonator mirror in order to achieve a maximum laser output power over a given lifetime of the laser radiation source.
The invention makes possible a comparatively simple, manageable calculation of a resonator mirror with a saturable absorber since it is based on the function of the individual components, the Bragg reflector and saturable absorber. The selection of the position of the single quantum layer within the threefold layer affords a simple possibility for influencing within wide limits the stability of a resonator mirror of this kind with respect to radiation.
Further, a comparatively simple process control also results for the layer construction by means of appropriate processes in thin-film technique, wherein the use of epitaxially grown layers is preferred at the present time.
The threefold layer can also have an optical thickness that is a whole-number multiple i of             λ      L        2    ,
wherein the advantages of the invention are retained. The threefold layer which is thicker by the factor i is required for protection of the single quantum layer primarily because of an improved surface passivization, where the selected value for i, generally 2 or 3, is sufficient.
The absorption behavior of the resonator mirror with the saturable absorber is adjusted by the selection of the thickness d3 of the single quantum layer and its position between the two layers, wherein one of the two layers has a minimum thickness of             λ      L        100    .
This minimum thickness should be greater than the thickness of the single quantum layer in every case. The single quantum layer must be located at a sufficient distance from the standing wave minimum and be adequately protected mechanically and chemically from adjoining media.
The absorption behavior of the single quantum layer essentially determines the pulse duration of a mode-synchronized laser in which this resonator mirror is used. The position of the single quantum layer within the two layers is determined according to the criterion of the desired or required stability of the resonator mirror with respect to lasers, wherein this position must be located far enough away from the standing wave minimum of the laser radiation so that the required saturably absorbing effect is retained in order to generate short laser pulses, e.g., in the picosecond range. The absorption behavior and therefore the pulse duration generated in a laser are adjustable in an especially favorable and reproducible manner through the selection of the position of the single quantum layer between the two enclosing layers inside the threefold layer. The pulse duration and absorption behavior in turn determine the power stability of the resonator mirror, wherein it is expected that longer pulses increase the power stability of the resonator mirror.
However, it has been shown that the characteristics and working parameters of the resonator mirror with the saturable absorber are highly dependent upon the respective installation conditions within a laser resonator (resonator design).
An advantage resulting from the invention consists in that the resonator mirror with the saturable absorber is optimized in cooperation with the laser resonator with respect to the specific requirements of the user by means of a simple, manageable step: the placement of the single quantum layer inside the threefold layer.
In practice, the following procedure is carried out: First, a resonator geometry is planned which contains at least the laser crystal, an output-coupling mirror and the resonator mirror (see Kxc3x6cher, W., Solid-State Laser Engineering, Vol. 1, Edition 4, Berlin, Springer Verlag 1996). With knowledge of the teaching according to the invention, few experiments in which the position of the single quantum layer inside the threefold layer is varied are necessary to determine the pulse durations and laser stabilities at different laser output powers. For a specific construction of a laser resonator, dependencies are determined and utilized for optimizing the position of the single quantum layer inside the threefold layer.
Experiments with a resonator design have shown that the shortest pulse lengths were achieved when the single quantum layer is arranged in the standing wave maximum of the laser radiation. However, the relatively low power stability of the resonator mirror was also determined in this position.
The single quantum layer is therefore preferably arranged outside an intensity maximum of the laser radiation. In practice, the invention utilizes a position of the single quantum layer inside the threefold layer between the standing wave maximum and the standing wave minimum of the laser radiation. With this dimensioning, an increase in the pulse duration was observed; however, this pulse duration can be further reduced through additional steps to be named hereinafter, so that sufficiently short pulses of the laser radiation in the picosecond range and a sufficient laser stability of the resonator mirror in the CW watt range are achieved.
A reduction in the pulse duration of the laser radiation is achieved by an anti-reflection coating which is applied to the threefold layer and which is designed for a laser wavelength xcexL. A further increase in the power stability of the resonator mirror is also achieved by means of the anti-reflection coating. The operation of the saturable absorber is neither resonant nor anti-resonant as a result of this anti-reflection coating.
Further, the pulse duration of the laser radiation is decreased in that the single quantum layer is applied as a low-temperature layer, wherein the pulse duration decreases as the selected growth temperature is reduced. In particular, the single quantum layer comprises one of the following material systems: indium gallium arsenide (InGaAs) or gallium arsenide antimony (GaAsSb) or gallium nitrogen arsenide (GaNAs).
For minimizing expenditure for the production of the resonator mirror with the saturable absorber, it is particularly advantageous when one of the materials used to construct the Bragg reflector is also used to construct the threefold layer. Layers of identical or structurally very similar materials are grown on one another epitaxially in a particularly favorable manner. The materials for the production of the Bragg reflector are especially suitable for this purpose. In this case, the threefold layer advantageously comprises a layer of the first material or of the second material of the Bragg reflector, a single quantum layer and a further layer comprising the first material or second material of the Bragg reflector. The single quantum layer, whose thickness d3 depends on the laser wavelength xcexL, is embedded between two layers of the first material with the index of refraction nH or between two layers of the second material with the index of refraction nL, wherein these layers with their thickness d1, d2 and d3 form the threefold layer which has a thickness of             λ      L              2      *              n        H              ⁢            xe2x80x83        ⁢          xe2x80x83        ⁢  or  ⁢      xe2x80x83    ⁢                    λ        L                    2        *                  n          L                      .  
The anti-reflection coating is also advantageous in this case, wherein its index of refraction is designed according to {square root over (nH)} or {square root over (nL)} and the anti-reflection coating has a thickness of                     λ        L                    4        *                              n            H                                ⁢          xe2x80x83        ⁢    or    ⁢                  xe2x80x83            ⁢              xe2x80x83              ⁢                  λ        L                    4        *                              n            L                                ,
depending on the adjoining material.
To increase the laser strength of the resonator mirror, the latter is fastened to a heat sink by its substrate. This has the purpose of ensuring a constant operating temperature of the saturable absorber, wherein a fine matching of the saturably absorbing layer to the respective laser wavelength xcexL is carried out by adjusting the reference temperature. This heat sink also ensures the required high constancy of the peak output over time.
An advantageous arrangement of this procedure consists in that the substrate is made of gallium arsenide (GaAs) and the Bragg reflector comprises individual layers, each of which has a thickness of   λ      4    *          n      GaAs      
for the first material with the refractive index nH with undoped gallium arsenide (GaAs) and       λ    L        4    *          n      AlAs      
for the second material with the lower refractive index nL with undoped aluminum arsenide (AlAs), and the single quantum-layer comprises indium-gallium arsenide (In1-xGaxAs). The Bragg reflector comprises 15 to 50 individual layers which form mirror pairs. The number of mirror pairs determines its reflectivity factor (see Orazio Svelto, xe2x80x9cPrinciples of Lasersxe2x80x9d, 4th edition, Plenum Press 1998). For example, a reflectivity of the resonator mirror of over 98% is achieved with 28 mirror pairs. In practice, it is always attempted to work with as few layers as possible. The characteristics of the material system comprising gallium arsenide/aluminum arsenide have been investigated to a sufficient extent that these materials can be epitaxially grown on the substrate of gallium arsenide in a comparatively simple manner with the required homogeneity of layer thicknesses and layer construction.
The dimensioning of the single quantum layer of indium gallium arsenide (In1-xGaxAs) for the respective laser wavelength is known from the literature (see, e.g., Wang, C. A., Hong, K.Ch., xe2x80x9cOrganometallic Vapor Phase Epitaxy of High-Performance Strained-Layer InGaAs-AlGaAs Diode Lasersxe2x80x9d, IEEE Journal of Quantum Electronics, Vol. 27, No. 3, March 1991). For a laser wavelength of 1064 nm, the thickness of the single quantum layer is about 7 nm and the gallium proportion x=67%.
The single quantum layer is embedded in two gallium arsenide layers, wherein these layers, together, must have the corresponding xcex/2 thickness. In practice, the layer construction is carried out in such a way that an interference-free layer construction and a technologically more favorable process run are ensured.
The absorption behavior and accordingly the pulse duration generated in a laser are adjustable in a particularly favorable and reproducible manner through the selection of the position of the indium-gallium-arsenide single quantum layer between the two gallium arsenide layers inside the threefold layer. The pulse duration and absorption behavior in turn determine the power stability of the resonator mirror. One of the gallium arsenide layers arranged inside the threefold layer acting as saturable absorber has a minimum thickness of about             λ      L        100    .
This minimum thickness is required so that the indium gallium arsenide layer functions as an absorbing single quantum layer and so that this layer, which is very sensitive to environmental influences, is adequately protected.
The gallium arsenide layer facing away from the Bragg reflector accordingly always serves as a protective layer for the indium gallium arsenide layer relative to the surrounding medium. In a special case, the indium gallium arsenide layer is embedded within two gallium arsenide layers with an optical thickness of xcexL/4. In this case, the indium gallium arsenide layer is located exactly in the standing wave maximum. This has the disadvantage of a maximum energy density at this location. However, this disadvantage is overcome in that the diameter of the beam bundle entering the resonator mirror is adjusted so as to be comparatively large; instead of a spot diameter of 10 xcexcm, spot diameters greater than 200 xcexcm can be adjusted. However, this is only possible in a very homogeneous layer construction which is benefited by the comparatively very simple layer construction and the comparatively small number of individual layers.
The indium gallium arsenide layer is advantageously a low-temperature layer. The growth temperature should be under 500xc2x0 C. in order to generate sufficiently short laser pulses. However, the aim of the present invention is not the generation of the shortest possible laser pulses as is desirable for applications in communications technology.
However, a low-temperature layer ensures that the saturable absorber also delivers sufficiently short laser pulses with an optimization of the layer construction with respect to its power stability which are advantageously in the range of 1 to 10 picoseconds for many technical applications. Examples of technical applications are materials processing or image projection by means of laser light.
The influence of the pulse duration of the laser light on image generation is described, for example, in WO 98/20385. It is stated therein that the so-called speckle phenomena can no longer be perceived by an observer when images are generated by laser light with a pulse duration in the picosecond range.
As was already described in general, an anti-reflection coating is advantageously applied to the outer gallium arsenide layer remote of the reflector. The anti-reflection coating is designed for a laser wavelength xcexL, wherein its index of refraction is calculated according to {square root over (nGaAs)} and used with nGaAs for laser wavelength xcexL. A reflection factor of less than 1% can be achieved without great effort and the calculated index of refraction need only be achieved approximately. The optical thickness of the anti-reflection coating is xcexL/4.
For laser wavelength xcexL=1064 nm, the anti-reflection coating is produced from a layer of silicone oxinitride or from a layer of silicone nitride. The anti-reflection coating delivers a higher intensity in the single quantum layer. With an anti-reflection coating on the resonator mirror with the saturable absorber, comparatively shorter pulse durations were measured.
A further advantageous arrangement of the invention consists in that the substrate is formed of indium phosphide (InP) and the Bragg reflector is formed of individual layers, each of which has a thickness of       λ    L        4    *          n      InGaAs      
for the first material with the refractive index nH with indium gallium arsenide (In1-yGayAs) and       λ    L        4    *          n      InP      
for the second material with the lower refractive index nL with indium phosphide (InP).
The single quantum layer also comprises indium gallium arsenide (In1-xGaxAs), wherein its layer thickness is in the range of 6 nm to 10 nm and its composition (x) is determined by the laser wavelength xcexL. In this case, also, the gallium proportion (x) determines the size of the band gap. The Bragg reflector in this case comprises 30 to 100 individual layers. The gallium proportion y is 47% in the indium gallium arsenide layers of the Bragg reflector in order to maintain the grating matching with the indium phosphide layers. This resonator mirror is suitable for laser wavelengths xcexL greater than 1.65 xcexcm.
In the present case, the single quantum layer is embedded between two layers of indium gallium arsenide or between two layers of indium phosphide.
A further advantageous arrangement of the invention consists in that the substrate comprises indium phosphide (InP) and the Bragg reflector comprises individual layers, each of which has a thickness of       λ    L        4    *          n      InGaAsP      
for the first material with the refractive index nH with indium gallium phosphide (In1-yGayAszP1-z) and       λ    L        4    *          n      InP      
for the second material with the lower refractive index nL with indium phosphide (InP), and the single quantum layer comprises indium gallium arsenide (In1-xGaxAs), wherein its layer thickness and composition are determined by the laser wavelength xcexL. In particular, the gallium proportion y and the arsenide proportion z are determined by the relationship y=0.4z+0.067z2 in order to achieve the grating matching between the indium gallium arsenide phosphide layers (In1-yGayAszP1-z) and the indium phosphide layers (InP) of the Bragg reflector. Depending on the gallium proportion, this resonator mirror is suitable for laser wavelengths xcexL greater than 1.3 xcexcm. However, since the difference in the index of refraction in the layer construction of the Bragg reflector is comparatively slight, more mirror pairs must be used in this case to achieve an equal reflectivity. Typically, 40 to 100 mirror pairs are necessary for functioning in a laser resonator.
In this case, as well, the single quantum layer is embedded between two layers of indium gallium arsenide phosphide or between two layers of indium phosphide.
The invention is described in the following with reference to drawings.